Known resin compositions employed in the field of printed circuit boards used in the formation of a permanent protective mask such as a solder resist or chemical plating resist comprise thermohardenable resins such as epoxy or melamine resins as the main component. Screen printing has been frequently used for the formation of patterns. The major object in the use of a solder resist is to limit the region to be soldered, to prevent the formation of a solder bridge, to prevent the corrosion of a conductor and to maintain the electrical insulating properties between conductors.
However, recent development in very large scale integrations have made it necessary to shorten the distance between conductors. Thus, it is now required to achieve higher electrical insulating properties as well as higher dimensional accuracy of, for example, a solder resist. However, screen printing inherently has a low degree of resolution and is accompanied by some problems such as skip and pinhole in the case of a highly viscous ink, or bleeding, running and sagging in the case of a less viscous ink. Therefore, it is impossible to satisfactorily accommodate the development of high-density printed circuit boards by screen printing.
Under these circumstances, a light-sensitive resin composition, which enables the formation of a pattern by photography wherein an image is formed by exposure followed by development, has a high sensitivity as well as a high degree of resolution, shows an excellent adhesion to a substrate and gives a film excellent in electrical and mechanical properties after hardening, attracts public attention today.
Further, it has been recently required to develop a light-sensitive resin composition capable of being developed with an alkaline aqueous solution or water, by taking the working environment and air- and water-pollution into consideration.
Dry-film type or liquid-type light-sensitive resin compositions capable of being developed have been known as a permanent protective mask for a printed circuit board for pattern-formation.
Examples of light-sensitive resin compositions of the dry film type include a light-sensitive resin composition comprising urethane di(meth)acrylate, a linear polymer compound and a photosensitizer disclosed in JP-A-57-55914 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application") (corresponding to U.S. Pat. No. 4,499,163) and another one comprising a (meth)acryl-modified resin of a specific novolak epoxy resin and a photosensitizer disclosed in JP-A-62-247353.
Generally speaking, however, light-sensitive resin compositions of the dry film type would frequently suffer from the formation of bubbles upon hot contact bonding and be unsatisfactory in thermal stability and adhesion. As JP-A-52-52703 (corresponding to U.S. Pat. No. 4,101,364) teaches, a specific process such as hot contact bonding under reduced pressure is required in order to overcome these problems. Furthermore, it is not always possible to establish a perfect thermal resistance or adhesion even though the above mentioned specific process is employed.
On the other hand, liquid light-sensitive resin compositions capable of being developed are suitable for, e.g., a permanent protective mask for a printed circuit board having narrow intervals of conductors. Such a liquid light-sensitive resin composition is applied as such onto a printed circuit board immediately before use and thus requires less steps. Furthermore, it can be applied to a highly uneven printed circuit board so as to form an excellent image.
However, these liquid light-sensitive resin compositions are disadvantageous in that the direct adhesion of a printed circuit board, to which a liquid light-sensitive resin composition have been applied, to a pattern mask followed by development causes stains on the pattern mask.
In order to overcome this problem, i.e., stains on a pattern mask, JP-A-57-164595 disclosed a specific process comprising placing a pattern mask at a specific distance apart from a printed circuit board, on which a liquid light-sensitive resin composition is applied as such without drying, hardening the liquid light-sensitive resin composition through exposure and then removing the unhardened liquid light-sensitive resin composition, which is different from a common process of applying a liquid light-sensitive resin composition onto a printed circuit board and drying it to thereby form a film. However this process is disadvantageous in that the surface of the applied liquid light-sensitive resin composition, which is not dried for film-formation, should be placed at a distance from the pattern mask, which lowers the degree of resolution. Furthermore, this process requires use of a specific device, which increases the cost.
Furthermore, JP-A-58-24144 and JP-A-59-2049 each disclose a process for the production of a solder resist which comprises applying a liquid light-sensitive resin composition onto a transparent and flexible substrate or pattern mask capable of transmitting active light in such a manner as to give a uniform thickness; immediately transporting the flexible substrate or pattern mask; pressing the surface of the substrate or pattern mask, on which the liquid light-sensitive resin composition has been applied, toward a printed circuit board under a constant pressure to thereby laminate the liquid light-sensitive resin composition onto the printed circuit board at a constant thickness; and then conducting the development through exposure by peeling the flexible substrate or pattern mask. However this process has some disadvantages. When the liquid light-sensitive resin composition applied on the flexible substrate or pattern mask at a uniform thickness is pressed toward the printed circuit board under a constant pressure, the unevenness of the printed circuit board causes the rolling-in of bubbles. As a result, the image thus formed involves a considerable number of bubbles. Further, this process is unsatisfactory from the viewpoints of thermal resistance and adhesion. Furthermore, it requires the use of a specific device, which increases the cost.
Furthermore, JP-A-61-102652 and JP-A-62-27736 each disclose a process for the production of a solder resist which comprises applying a liquid light-sensitive resin composition onto a printed circuit board, pressing a transparent and flexible substrate or pattern mask toward the liquid light-sensitive resin composition applied on the printed circuit board under constant pressure, exposing the liquid light-sensitive resin composition to light through the pattern mask, and developing the same after peeling the flexible substrate or pattern mask. However, this process also suffers from some problems. For example, it requires specific devices for pressing the flexible substrate or pattern mask under constant pressure and for the exposure, which increases the cost.
When the above-mentioned liquid light-sensitive resin composition is not dried but exposed to light as such according to the resist pattern, the liquid light-sensitive resin composition, which comprises a liquid prepolymer containing a number of reactive monomers, is inferior in properties including acid resistance and chemical resistance to those observed in common methods.
A process is known for the production of a solder resist which comprises applying a liquid light-sensitive resin composition, drying the same by heating, adhering a pattern mask thereto and developing the same through exposure. Examples of liquid light-sensitive resin compositions used in this process include the one using a reaction product obtained by reacting a novolak epoxy resin with an unsaturated monobasic acid as disclosed in JP-A-62-7773 and JP-A-62-7774 (both corresponding to EP-A-0207188) or the one disclosed in JP-B-52-34935 (the term "JP-B" as used herein means an "examined Japanese Patent Publication") (corresponding to U.S. patent application Ser. No. 06/426,877 filed on Dec. 20, 1973).
On the other hand, it has been recently required to develop a light-sensitive resin composition capable of being developed with an alkaline solution or water, by taking the working environment and air- and water-pollution into consideration. Examples such resin composition include a liquid resist ink composition disclosed in JP-A-61-243869 which comprises a novolak epoxy compound and a reaction product obtained by reacting an unsaturated monocaboxylic acid with a polybasic acid anhydride, a liquid resin composition disclosed in JP-A-62-187722 which comprises a bisphenol epoxy resin having an unsaturated group and a carboxyl group as the major component, the composition which comprises the urethane compound having a terminal unsaturated group at a terminal of the molecule thereof and a terminal carboxyl group at the other terminal as disclosed in JP-A-57-164595 (corresponding to U.S. patent application Ser. No. 06/225810) and the composition which comprises a carboxy-modified epoxy (meth)acrylate as disclosed in U.S. Pat. No. 4,789,620. Further, the similar modified product of the novolak resin or epoxy resin as mentioned above is disclosed, for example, in JP-B-52-34935 and U.S. Pat. No. 3,980,483 or U.S. Pat. No. 4,789,620, respectively. Furthermore, light-sensitive resin compositions and components which can be used therefor have been disclosed in prior arts. For instance, a polymerizable compound having an ethylenic unsaturated double bond is disclosed, for example, in JP-B-52-34935 (corresponding to U.S. patent application Ser. No. 06/426,877 filed on Dec. 20, 1973), and a specific compound thereof is disclosed, for example, in U.S. Pat. Nos. 4,322,491, 4,604,342, OLS 2064079 and JP-A-61-228007; an epoxy compound is disclosed, for example, in JP-A-61-243869 and U.S. Patent 4,485,166; a photopolymerization initiator is disclosed, for example, in JP-A-53-133428, JP-B-57- 1819, JP-B-57-6069 and U.S. Pat. No. 3,615,455; an catalyst is disclosed, for example, in JP-A-61-243869.
However, these liquid light-sensitive resin compositions are also unsatisfactory in sensitivity and in the electrical properties and chemical resistance of the films obtained after hardening.